1. Field of the Invention
The present invention relates to a light-emitting device and a lighting device. In particular, the present invention relates to a light-emitting device and a lighting device to which a light-emitting element utilizing electroluminescence is applied.
2. Description of the Related Art
In recent years, research has been conducted on light-emitting devices including organic electroluminescence (hereinafter, referred to as EL) elements, which utilize EL, as self-luminous light-emitting elements. An organic EL element can provide surface light emission with a large area because a pair of electrodes and a light-emitting layer are formed in film forms. Since an organic EL element can provide surface light emission, application of the organic EL element to a light source such as a lighting device or a backlight source of a liquid crystal display device has been attempted.
For obtaining light emission of a desired color with the use of an organic EL element, for example, an organic EL element including a light-emitting layer formed of a plurality of stacked layers whose emission colors are different from each other (which have different emission spectra) is formed. The emission spectrum of an organic EL element including such a light-emitting layer corresponds to a combination of a plurality of emission spectra; thus, light emission of a desired color can be obtained.
For obtaining white light emission, for example, a white-light-emitting organic EL element including a light-emitting layer formed of a plurality of stacked layers is used; examples of the white-light-emitting organic EL element include a three-wavelength-type white organic EL element in which layers emitting red light, green light, and blue light (three primary colors of light) are stacked, and a two-wavelength-type white organic EL element in which layers emitting light of complementary colors (e.g., blue and yellow orange) are stacked. In each of these elements, a plurality of emission spectra is combined, so that white light emission is obtained.
In order that an organic EL element emits light of a desired color, the peak intensities of respective emission spectra, the shapes of the spectra, and the balance therebetween are optimized. For example, Patent Document 1 discloses a technique for obtaining objective white light emission by setting the thickness of an organic light-emitting layer, the proportion of a guest material in a host material, the thickness of a hole-transport layer, and the thickness of an electron-transport layer.
The color tone of light emitted from a lighting device is required to be suitable for the intended use in some cases. For example, the color temperature for use in homes and that in offices are different from each other. Further, for display in retail stores, not only white light emission but also light emission of other various colors is required in some cases.